This invention relates to charge coupled devices (CCDs) and, more particularly, to a low noise configuration for amplifying CCD output charge. This invention was made with government support under Contract No. W-7405-ENG36 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
CCDs form the core of modern electronic imaging. CCDs are used, e.g., in camcorders, the Hubble telescope, and in many other scientific applications, as a quantitative imager. For scientific applications, the emphasis is often on ultra-low noise to allow a few photons per pixel to be detected. Slow frame rates of 0.01 to 0.1 per second, with bandwidths of 5 to 200 kHz can be used to minimize noise. But the developing markets in engineering and process control require faster frame rates of 1 to 10 per second, accepting noise of 10 to 40 photons per pixel to provide the increased speed.
Instrument amplifier designs, such as CCD output circuits, often encounter a small voltage input across a high impedance, where the high impedance is usually caused by a small capacitance. For DC instruments an ultra-high input impedance may be the optimum noise amplifier to use, but this is not the case for high speed or broadband applications. Therefore, the common desire to use a high equivalent load impedance for maximum voltage transfer may not be valid where active devices, such as transistors and FETs, are used in the circuit design.
In general, a high input impedance for FET semiconductor devices is not optimum, but is often chosen. Such a prior art CCD amplifier circuit is shown schematically in FIG. 1. CCD 12 outputs a charge signal, i.e., a current, to FET 14 connected in source follower configuration. CCD 12 depicts only a CCD element up to its output node, with shunt capacitance C.sub.o at the output node. FET 14 has gain g.sub.m and shunt capacitance C.sub.gs. An intermediate signal is output from FET 14 to amplifier 16 connected as a high impedance load to the source of FET 14. However, the noise resulting from the feedback resistance R.sub.f for the second stage amplifier 16 is often much larger than the intrinsic noise of FET 14 so that minimum overall noise at readout rates below about 20 Megapixels/sec would be achieved by retaining the FET in its highest current gain, or common source, configuration.
For FET devices the device transconductance gain g.sub.m is proportional to the input capacitance C.sub.gs. At high frequencies, where C.sub.gs begins to dominate circuit impedance, the ratio g.sub.m /C.sub.gs is a figure of merit .eta. for the device, such that g.sub.m =.eta.C.sub.gs. To maintain a high input impedance, C.sub.gs must be kept to a small capacitance value. But gain g.sub.m is decreased at a given .eta. by reducing C.sub.gs. Gain g.sub.m can be maintained by increasing drain current (g.sub.m .varies.I.sub.d ), but this results in concomitant heating of the FET and increased shot noise in the FET.
An additional aspect of FET performance important to CCD amplifier improvements is noise: Johnson, or white, noise and pink, or flicker, noise. White noise can be reduced by reducing the operating bandwidth. But flicker noise has an amplitude spectral density that is inversely related to frequency so that flicker noise sets a "floor" level for noise reduction. In the case of MOS FET devices, it has been observed that flicker noise reduces in amplitude and corner frequency as the gate length (L) is increased. But FET gain-bandwidth is undesirably reduced by increasing L.
These CCD output noise problems are addressed by the present invention by alleviating the need for slow and low frequency operation. This CCD output amplifier design has improved output noise performance.
Accordingly, it is an object of the present invention to provide an overall FET/amplifier design for use with CCDs that reduces output noise while maintaining output bandwidth at high sample rates.
Another object of the present invention is to minimize the effect of flicker noise while providing an acceptable gain-bandwidth for the circuit.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.